tion facilities (greater than 100 MWp worth of solar modules per year). This size limitation does not allow the economy-of-scale benefits for the solar cell production. Many companies use multiple technologies. The current cost of solar modules is in the range of $3 to $6 per peak watt (Wp). For solar cells to be competitive with the conventional technologies for electricity production, the module cost must come down below $1/Wp. Table G-9 provides cost estimates of producing electricity as well as hydrogen calculated by the committee. In the current scenario, with a favorable, installed cost of about $3.285/Wp, the electricity cost is estimated to be about $0.319/kWh (scenario Dist PV-C of Chapter 5). For a futuristic case with all of the expected technology and production advances, the anticipated installed cost of $1.011/ Wp provides electricity cost of $0.098/kWh (scenario Dist PV-F, Table E-49 in Appendix E). While this target is attractive for electricity generation, it does not produce hydrogen at a competitive cost.

Energy is consumed in the manufacture of solar modules. It has been estimated by NREL that for a crystalline silicon module, the payback period of energy is about 4 years. For an amorphous silicon module this period is currently about 2 years, with the expectation that it will eventually be less than 1 year.

Future Technology

Photovoltaic Cells

Various developments are likely to improve the economic competitiveness of solar technology, especially for thin-film technology. The current research on microcrystalline silicon deposition techniques is leading to higher efficiencies. Techniques leading to higher deposition rates at moderate pressures are being developed (Schroeder, 2003). Better barrier materials to eliminate moisture ingress in the thin-film modules will prolong the module life span. Robust deposition techniques will increase the yield from a given type of equip

ment. Inline detection and control methods will help to reduce the cost. Some of this advancement will require creative tools and methods.

The committee believes that installed costs of roughly $1/ Wp are attainable. Material costs are quite low, but substrate material, expensive coating equipment, low utilization of equipment, and labor-intensive technology lead to high overall costs. It is expected that in the next decade or two, improvements in these areas have a potential to bring the cost much below $1/Wp. World-class plants with economies of scale will further contribute to the lowering of cost. For crystalline-silicon-wafer-based technology, the raw material costs by themselves are almost $1/Wp. However, improvements in operating efficiency, the cost of raw materials, and reduced usage of certain materials are expected to bring overall cost in the neighborhood of $1/Wp.

A concept that has been proposed is the dye-sensitized solar cell, also known as the Grätzel cell (O’Regan and Grätzel, 1991). A dye is incorporated in a porous inorganic matrix such as TiO2, and a liquid electrolyte is used for positive charge transport. Photons are absorbed by the dye, and electrons are injected from the dye into n-type titania nanoparticles. The nanoparticles of titania are fused together and carry electrons to a conducting electrode. The dye gets its electron from the electrolyte, and the positive ion of the electrolyte moves to the other electrode (Grätzel, 2001). This type of cell has a potential to be low-cost. However, the current efficiencies are quite low, and the stability of the cell in sunlight is very poor. Research is needed to improve performance at both fronts.

Another area of intense research is that on the integration of organic and inorganic materials at the nanometer scale into hybrid solar cells. The current advancement in conductive polymers and the use of such polymers in electronic devices and displays provides the impetus for optimism. The nano-sized particles or rods of the suitable inorganic materials are embedded in the conductive organic polymer matrix. Once again, the research is in the early phase and the current efficiencies are quite low. However, the production of solar cells based either solely on conductive polymers or on hybrids with inorganic materials has a large potential to provide low-cost solar cells. It is hoped that one would be able to cast thin-film solar cells of such materials at a high speed, resulting in low cost.

Regarding production costs, all of the technologies discussed so far convert solar energy into electricity and use the electricity to generate hydrogen through the electrolysis of water. Since PV cells produce dc currents, the electric power can be directly used for electrolysis. As discussed in the section above on electrolyzers, considerable cost reductions are anticipated, which will lower the cost of hydrogen from solar cells. These cost reductions will be particularly valuable for solar cell electricity because the low usage factor associated with PV modules also contributes to the low usage of electrolyzers. This contributes heavily to the cost of hydro-

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